1. Field of the Invention
The present invention relates to a method for producing a negative electrode for a non-aqueous electrolyte secondary battery, and a negative electrode for a non-aqueous electrolyte secondary battery obtained by the method, and particularly to formation of a negative electrode material mixture layer which is less likely to separate or fall from a negative electrode current collector.
2. Description of the Related Art
A non-aqueous electrolyte secondary battery includes, as basic constituent elements, a positive electrode plate, a negative electrode plate, a separator which insulates the positive electrode plate and the negative electrode plate, and a non-aqueous electrolytic solution which enables migration of ions between the positive electrode plate and the negative electrode plate. The positive electrode plate is composed of a positive electrode current collector and a positive electrode material mixture layer formed on the surface of the positive electrode current collector, while the negative electrode plate is composed of a negative electrode current collector and a negative electrode material mixture layer formed on the surface of the negative electrode current collector.
For example, the positive electrode plate of a lithium ion secondary battery as a typical non-aqueous electrolyte secondary battery is obtained by applying a slurry-like paste, which is prepared by dispersing a positive electrode active material such as lithium cobaltate, a conductor and a binder resin in a suitable dispersion solvent, on a positive electrode current collector such as an aluminum foil, and drying the slurry-like paste to form a positive electrode material mixture layer, followed by rolling.
Also, the negative electrode plate of the lithium ion secondary battery is obtained by applying a slurry-like paste, which is prepared by dispersing a negative electrode active material capable of absorbing and emitting lithium upon charging and discharging and a binder resin in a suitable dispersion solvent, on a negative electrode current collector such as a copper foil, and drying the slurry-like paste to form a negative electrode material mixture layer, followed by rolling. As the negative electrode active material, a carbon material such as non-graphitizable carbon is preferably used.
A lead as a current terminal is attached to each of the positive electrode plate and the negative electrode plate. An electrode group is formed by winding or laminating the positive electrode plate and the negative electrode plate with a separator interposed therebetween. A lithium ion secondary battery is assembled by encasing the electrode group in a battery case, injecting a non-aqueous electrolytic solution therein and sealing the battery case. As the non-aqueous electrolytic solution, a solution prepared by dissolving a lithium-based compound in the nonaqueous solvent is used.
So as to maintain performance of the battery, it is required for the binder resin contained in the negative electrode material mixture layer to be chemically stable in an electrolytic solution, to be less likely to elute in the electrolytic solution, to be free from swelling caused by absorption of the electrolytic solution and to be applicable to a negative electrode current collector in a proper thickness. In a step of producing a negative electrode plate, after a negative electrode material mixture layer paste is applied on the negative electrode current collector and dried to form a material mixture layer, a rolling step by press working using a roll press and a flat plate press is commonly performed so as to increase adhesion between a current collector and a material mixture layer and to improve density and homogeneity of the material mixture layer. After the rolling step, post-processing such as a cutting step and an assembling step is performed. In the post-processing, the binder resin is required to have flexibility and high adhesion with the current collector so as to prevent separation such as detachment or falling of the negative electrode material mixture layer from the negative electrode current collector and the occurrence of cracks.
Also, the amount of the binder resin is preferably a small amount so as to improve performances of the battery for the following reason. Namely, when a large amount of the binder resin is added, the surface of the negative electrode active material is coated with the binder resin and thus discharge characteristics deteriorate. In order to maintain sufficient adhesion to the current collector using a small amount of the binder resin, a binder resin having a low glass transition temperature and high tackiness is preferably used. However, when such a binder resin having high tackiness is used, the formed material mixture layer may adhere onto a press mold upon press working, thereby a portion of the material mixture layer separating and falling from the current collector, and thus yield rate decreases.
Some proposals on binder for improving adhesion between the material mixture layer and the current collector resin have hitherto been made.
For example, Japanese Patent No. 3,101,775 (Patent Document 1) discloses a negative electrode for a secondary battery, containing a carbon material as an active material in which a negative electrode active material is bound by a binder resin containing, as a main component, a styrene/butadiene latex which contains 40 to 95% by mass of butadiene and 75 to 100% of gel content. Since the binder resin has a low glass transition temperature (Tg), a material mixture paste is applied in a rubber state when applied at a room temperature and the rubber state is also maintained in a post-processing step. Since a binder resin has a high adhesive strength when it exists in a glass state at a temperature lower than the Tg, adhesion of a binder resin in the glass state is not sufficiently exhibited when the post-processing step is performed in the rubber state. In this case, it becomes more likely for the material mixture layer to separate or fall from the current collector. Particularly, when processing is performed at a high application speed and a high rolling speed, there is a remarkable tendency that a sufficient adhesive strength is not attained. When the adhesive strength is increased by increasing the amount of the binder resin so as to suppress separation or falling, there arises a problem that, when the surface of the negative electrode active material is coated with the binder resin, discharge characteristics deteriorate and the capacity under high temperature conditions remarkably decreases.
Also, Japanese Unexamined Patent Publication (Kokai) No. 2000-67871 (Patent Document 2) discloses a negative electrode for a non-aqueous electrolyte secondary battery, using a mixture of one or more kinds of binder resins selected from a binder resin (A) selected from styrene-butadiene copolymers in which the bound styrene content is 20% by mass or more and 70% by mass or less based on the entire styrene-butadiene copolymer, and a resin (B) selected from a styrene-butadiene copolymer and polystyrene in which the bound styrene content is 80% by mass or more and less than 100% by mass.
Patent Document 2 also discloses that, when the binder resin (A) and the binder resin (B) are used, a negative electrode active material is scarcely coated with the binder resin (B) and the negative electrode active materials are bonded by the binder resin (A) through the binder resin (B) and thus a negative electrode having a high peel strength of a material mixture layer and excellent handling properties is obtained, and that it becomes possible to improve discharge characteristics of the non-aqueous electrolyte secondary battery at low temperature. However, in the negative electrode, it is impossible to sufficiently exhibit characteristics in the rubber state of the binder resin (A) having a low Tg and characteristics in the glass state of the binder resin (B) having a high Tg. Namely, since the adhesive strength largely depends on the binder resin (A) having a low Tg, a sufficient adhesive strength to the current collector cannot be obtained unless a large amount of the binder resin is used in total.
Also, Japanese Unexamined Patent Publication (Kokai) No. 2004-247184 (Patent Document 3) discloses that the temperature of a mixture of a positive electrode active material, a conductive material, a binder resin and an organic solvent during mixing and stirring is controlled to a temperature range where the binder resin dissolved in the organic solvent is not precipitated as a solid and also the temperature of a mixture of a positive electrode active material, a conductive material, a binder resin and an organic solvent during mixing and stirring is preferably controlled within a range from 30 to 60° C. in a method for producing a positive electrode material mixture paste, which comprises the steps of mixing a positive electrode active material, a conductive material, a binder resin and an organic solvent, followed by stirring. This technique aims at enhancing stability of paste viscosity so as to improve productivity of the positive electrode. Although coatability is improved by enhancing stability of paste viscosity, the effect of suppressing falling or separation of the material mixture layer from the current collector is not obtained thereby.